The present invention relates to an ion selective electrode for use in measuring a concentration of specific ion contained in a sample liquid.
Heretofore, various kinds of ion selective electrodes have been known. The inventors have also proposed an ion selective electrode comprising a tubular member made of electrically insulating material, a silicon wafer substrate applied on one end of the tubular member and an ion sensitive film applied on an outer surface of the silicon wafer substrate.
FIG. 1 is a cross sectional view showing an embodiment of such an ion selective electrode. The ion selective electrode 1 comprises a small tube 2 having a rectangular cross section and made of electrically insulating material such as plastics, and an ion sensitive body 3 applied to one end of the tube 1 by means of adhesive layer 4. As illustrated in FIG. 2, the ion sensitive body 3 comprises a silicon wafer substrate 5, an ion sensitive film 6 applied on the outer surface of the substrate 5 and an electrode film 7 formed by depositing Al and CrCu layers successively on the inner surface of the substrate 5. The ion sensitive body 3 is secured to one end of the tube 2 by the adhesive layer 4 in such a manner that the ion sensitive film 6 is exposed to the sample liquid to be measured. The adhesive layer 4 must be formed by highly insulating and waterproof material and may be formed by silicon RTV adhesive manufactured and sold by Shinetsu Silicon Co., Ltd. In order to reinforce the coupling between the tube 2 and the ion sensitive body 3, it is preferable to apply a primer onto their surfaces to be cemented. When the electrode 1 is used as a pH electrode, the ion sensitive film 6 may be constituted by a thin film of silicon nitride (Si.sub.3 N.sub.4), aluminum oxide (Al.sub.2 O.sub.3) or tantalum pentoxide (Ta.sub.2 O.sub.5) applied on the outer surface of silicon wafer substrate 5 by means of CVD (chemical vapor deposition) or sputtering method, or may be constituted by a thin film of glass such as Corning 015 (SiO.sub.2 72.2%--CaO 6.4%--Na.sub.2 O 21.4%) sputtered on the silicon substrate 5. Further, when the ion selective electrode 1 is used as Na electrode, a thin film of glass of Na.sub.2 O 11%--Al.sub.2 O.sub.3 18%--SiO.sub.2 68% may be sputtered on the silicon substrate 5 to form the ion sensitive film 6. To the electrode film 7 is connected a core conductor 8 of a signal wire 9 by means of solder 10, said signal wire 9 being extended via a cap 11 made of insulating material, which cap is tightly clamped onto the other end of tube 2. The core conductor 8 may be connected to the ion sensitive body 3 by means of an electrically conductive adhesive agent such as a silver paste. In such a case, the electrode film 7 may be dispensed with.
In the known ion selective electrode, since the ion sensitive film 6 is formed on the silicon substrate by CVD or sputtering method, the ion sensitive film is not or hardly formed on a side edge 5a of the silicon wafer substrate 5 and thus, the silicon wafer substrate 5 might be exposed to the sample liquid at its side edge. Therefore, in the known electrode 1 the side edge 5a of silicon wafer substrate 5 is also covered with the adhesive layer 4.
However, in practice, it is very difficult to apply the adhesive agent uniformly onto the side edge 5a of substrate. If a part of the side edge 5a of substrate 5 is exposed to the sample liquid, the sample liquid might penetrate the silicon substrate 5 and the electrode 1 might be broken in a short time. Contrary to this, if the thick adhesive layer 4 is applied, a part of the applied adhesive agent might cover a part of the ion sensitive film 6 and an effective surface area of the ion sensitive film 6 might be reduced. Then, if the size of the electrode 1 is small, an electric impedance of the electrode 1 becomes too high and thus, a response of the electrode is deteriorated and the electrode is affected extremely by noise. This limits the miniaturization of the ion selective electrode to a great extent.